The present application relates to and incorporates herein by reference Japanese Patent Application No. 2000-287252 filed on Sep. 21, 2000.
The present invention relates to an alternator. More particularly, the present invention relates to a three phase alternator having a cooling fan for a vehicle.
In a conventional alternator disclosed in JP-Y2-62-16776, a cooling fan has a plurality of fan blades alternately combined with diagonal-flow-type blades and centrifugal-flow-type blades on an outer circumference thereof. The cooling fan blows cooling air introduced into a housing in an axial direction as well as in a radial outward direction, thereby increasing cooling of a three-phase armature coil, an field coil and resultant output power of the alternator. When the cooling fan is rotated inside a stator, wind noises are caused by the diagonal-flow-type blades and the centrifugal-flow-type blades, and interfere with each other, resulting in wind interference noise. This sound has a specific order of frequency proportional to a divisor or a multiple number of the numbers of the diagonal-flow-type blades or the centrifugal-flow-type blades.
According to U.S. Pat. No. 5,763,968 (JP-A-9-289756), in order to restrict the wind interference noise, the number of diagonal-flow-type blades and the number of centrifugal-flow-type blades are determined not to be divisible by the other number, and to have no common divisor.
However, in JP-Y2-62-16776 and U.S. Pat. No. 5,763,968, each of the diagonal-flow-type blades is positioned between adjacent two shoulder portions of claw-pole pieces which define an air passage, in order to increase cooling capacity. Therefore, during a rotation of a rotor, each of the diagonal-flow-type blades blows cooling air into each air passage and wind noises are increased. Moreover, each of the diagonal-flow-type blades are provided at an axial end of each air passage so that the number of the air passage and the number of the claw-pole pieces are equal. As the number of the diagonal-flow-type blades is equal to the number of the claw-pole pieces, wind noises caused by the diagonal-flow-type blades and the claw-pole pieces interfere with each other. Thus, a wind interference noise harsh on the ears is generated at a specific order of frequency.
According to one aspect of the present invention, a cooling fan has a plurality of diagonal-flow-type blades and a pole core has a plurality of claw-pole pieces. One of the numbers of the diagonal-flow-type blades and claw-pole pieces is not divisible by the other, and both of the numbers have no common divisor other than one. By arranging a combination of the numbers of the diagonal-flow-type blades and the claw-pole pieces, an interference of wind noises caused by the diagonal-flow-type blades and the claw-pole pieces during operation is restricted, thereby suppressing noises harsh on the ears.